Traditional total hip replacements involve inserting a stem of a femoral implant into the medullary canal of a patient's femur after the femur has been resected at the distal end of the femoral neck. The stem is usually tapered such that its sides gradually converge from a wider proximal end to a narrower distal end. This configuration allows the stem to fill the majority of the medullary canal as the femur gradually narrows in a distal direction and this helps to anchor the implant in the femur. A rounded tip is provided at the distal end of the stem and a femoral neck and head is provided at the proximal end. The femoral head is constituted by a spherical ball configured for location within a corresponding acetabular cup.
In recent years, more conservative approaches, such as hip resurfacing, have been employed rather than total hip replacement as described above. In this case the aim is to save as much healthy bone as possible and so the femur is preferably resected towards the proximal end of the femoral neck or even through the lower portion of the femoral head. The femoral head of the implant in the later case comprises a part-spherical exterior surface (configured for location within a corresponding acetabular cup) and an interior surface shaped to locate the femoral head on the remaining resected bone of the femur. This generally requires that the stem is relatively narrow at its proximal end so that there is room for sufficient healthy bone to be received within the profile of the femoral head to secure it in place. However, this configuration does not allow the stem to fill the majority of the medullary canal to anchor the implant in the femur.
Granted UK patent number GB2388321 discloses an improved stem profile conceived by the present inventor, which is suitable for use in hip resurfacing surgery, and which aims to address the above-mentioned problem. This stem profile has, at a proximal end thereof, a section having an external surface of frustoconical or generally frustoconical form. A base portion is provided at the maximum diameter of the section, which extends substantially axially away from, and is substantially the same diameter as, the end of the resected head into which the stem is to be inserted, in use. Whilst the shape of this stem is advantageous in transferring longitudinal loads through the femur after bone in-growth into the porous coating of the frustoconical section has occurred (usually a few weeks after surgery), loading before the bone has grown into the porous area has been found to be troublesome. More specifically, it has been observed that if patients ignore instructions to use crutches for two months immediately following surgery in order to lessen the load through the hip joint, the frustoconical section may act like a log splitter and break open the base of the femoral head and femoral neck. In such a case, revision surgery using conventional total hip replacement techniques, as described above, may be required.
The present inventor has devised an improvement to the stem profile disclosed in GB2388321 in order to try to stop the stem from being rammed into the base of the femoral head if the patient takes too much load before bone in-growth has occurred. This improvement consists of a flange extending circumferentially around the base of the frustoconical section to abut the periphery of the resected femur. However, the use of partial weight-bearing crutches for the first two months after surgery is still insisted upon as a precaution.
To add to the above, it has been found that the thick stem of the implant disclosed in GB2388321 can touch or come close to the bone of the femoral neck cortex, particularly on the inner (medial) side. This causes the bone in this contact region to preferentially lock onto the stem such that loads are passed down through the stem, bypassing the proximal bone in the femoral head and upper part of the femoral neck, and transferring to the contact region further down the femoral neck. Since bone responds to loads placed upon it and wastes away if it is not normally loaded, the bone in the base of the femoral head and upper part of the femoral neck will become thin and weak in this instance. This phenomenon is commonly known as stress shielding and is clearly an undesirable effect.
A further problem commonly encountered is that the bone into which the stem is inserted is not homogenous. This is particularly the case when dealing with arthritic patients. Usually the bone on one side of the femur is more dense than the bone on the opposite side, which is much softer. This has the effect that, when the stem of the implant is being inserted using a typical interference or press-fit technique, the denser bone does not compress as much as the softer bone and the path of insertion of the implant alters such that the implant moves away from the denser bone towards the softer bone. This results in a skew insertion (mis)alignment whereby the proximal end of the stem does not sit as intended within the recess prepared for it. This leads to stress concentration in the adjacent bone with a risk of fracture or implant loosening.
It is therefore an aim of the present invention to provide an improved femoral implant.